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custom implant the trochlear design and
positioning is not compromised as in
conventional techniques, by the variation of the
femoral component positioning guided by
flexion/extension gap balancing. The patellar
resurfacing debate remains open, but more
natural tracking should allow sparing of the
native patellar surface more frequently.
Fixation
A strong and harmonious fixation is usually
achieved in most of the patients with modern
designs. The fixation may nevertheless be
challenged in some situations. Typically in
overweight females with small joints the
fixation interface is reduced and fixation can be
compromised. This can also occur when a
residual deformity is present, especially a varus
deformity. The use or addition of longer stems
or fins is required in these situations. But the
reduced surface resulting from the cut, or the
lack of metaphyseal volume, or the presence of
a narrow diaphysis can make the insertion of
these extensions challenging. The shaft
alignment may also be challenging since it is
not always centered on the cut due to the local
anatomy or in relation to the obliquity of the
cut in both frontal and sagittal planes. In
customized knees this can be anticipated, and
the additional fixation extensions or devices,
can be aligned and proportioned accordingly.
The use of more proportional implant thickness
allows the reduction of the bone resection in
smaller patients, thus offering a wider and
stronger bony site, typically on the tibial side.
The femoral and inter-condylar boxes can also
be reduced in order to maximize bone sparing
whilst providing better fixation.
How to do it?
Technique
The last but not least surgical challenge is the
insertion of the implant. In customized implants
there is no role for traditional instrumentations
or intra-operative on the spot decision-making.
The whole procedure and specific adjustments
must have been anticipated during the planning
and the implant design phase that will generate
the patient specific cutting guides. The actual
alignment and the various contributions to the
deformity, including wear, ligament imbalance
and native deformity must be determined as
accurately as possible. From this analysis, the
reducibility of the deformity must be calculated
as accurately as possible to approximate the
final limb alignment. So far there is no absolute
way to predetermine the final alignment, this is
why there should be some degree of patient
selection and some room for intra-operative
adjustments.Thereducibilityofthepreoperative
alignment can be estimated on stress x-rays and
the overall analysis of the deformity based on a
3D model extracted from a CT-scan. Because
the femoral implant is the key element in
determining the future kinematics of the joint,
its position cannot be adjusted much during the
surgery, whereas on the tibial side it is easier to
perform fine-tune by adjusting the cut in depth
and direction. This option must be implemented
in both the tibial design and the instrumentation
(fig. 6).
Fig. 6: Accuracy of a personalized femoral cutting
guide along with the bony model that allows a final
matching check before realizing the bone
resection.